This paper presents a bibliographical survey of the work carried out to date on the solid state transformer (SST). The paper provides a list of references that cover most work related to this device and a short discussion about several aspects. The sections of the paper are respectively dedicated to summarize configurations and control strategies for each SST stage, the work carried out for optimizing the design of high-frequency transformers that could adequately work in the isolation stage of a SST, the efficiency of this device, the various modelling approaches and simulation tools used to analyze the performance of a SST (working a component of a microgrid, a distribution system or just in a standalone scenario), and the potential applications that this device is offering as a component of a power grid, a smart house, or a traction system.

Solid state transformer technologies and applications: A bibliographical survey

The low-voltage ride-through (LVRT) capability is a critical requirement of grid-connected converters of renewable energy resources to uphold the stability of the power system. The step-up transformer that connects the power converter to the grid is exposed to sudden voltage changes throughout the LVRT, and the resulting magnetic flux deviation often leads to severe inrush current when the grid fault clears. The inrush current exerts significant mechanical forces in the transformer and damages its insulation. This paper proposes a control method of the power converter to reduce the risk of inrush current by mitigating the magnetic flux deviation during the LVRT operation. The proposed method also manages the output current of the power converter within the capacity of semiconductor devices to reduce their stress while performing the proposed flux compensation and other LVRT operations. The reliability of the power converter and the transformer can be improved by the proposed control method.

A Transformer Inrush Reduction Technique for Low-Voltage Ride-Through Operation of Renewable Converters

The energization of power transformers is a frequent operation in electric arc furnaces (EAFs). Magnetization inrush currents with 10–50 times the rated transformer current can flow during several line cycles. The magnitude of the inrush currents depends on parameters like the point on wave of voltage energization, residual flux in the core, transformer size, and utility systems characteristics. This momentary high current results in system disturbances and reduction of the transformer life cycle. In this paper a scheme to mitigate large inrush currents using controlled voltage ramp soft-starter-based circuit is presented. Simulation and experimental results prove the effectiveness of the soft-starter-based controlled energization.

Mitigation of Electric Arc Furnace Transformer Inrush Current Using Soft-Starter-Based Controlled Energization

A scheme of restraining transformer inrush current based on control by closing voltage amplitude is proposed based on analyzing the influence factors of transformer inrush current and deriving the mathematical relationship between inrush current and influence factors. In this scheme, in order to making sure that the magnetic flux produced by the transformer no-load closing does not exceed saturated magnetic flux, the voltage amplitude should be controlled by selecting the appropriate curve, thus attaining the goal restraining the transformer inrush current. The control function of voltage amplitude is derived from the magnetic flux inequality and the attenuation law of the transient flux component, and the upper and lower limitation of initial value and slope in the voltage amplitude control function is deduced.

Transformer inrush current restraining scheme based on closing voltage amplitude controlling method

The low-voltage ride-through (LVRT) capability has become one of the most important issues since more renewable and distributed energy resource are installed in the grid. The LVRT capability means the converter should remain grid-connected and inject required current to support gird voltage. On the other hand, the sudden reduction of grid voltage may result in magnetic flux deviation in the step-up transformer as grid voltage sag occurs. The deviation of magnetic decays very slowly due to the high efficiency transformer, and it would lead to inrush current as grid voltage restores to normal level. This paper proposes a method to mitigate the magnetic flux deviation and limit the peak current without excelling the current capability of semiconductor devices by using a close-loop current control during LVRT operation. The proposed method can manage the converter's output peak current and reduce the risk of inrush current as grid voltage restores.

A transformer inrush reduction technique for low-voltage ride-through operation of renewable converters

Magnetizing inrush current in transformer results from any abrupt changes of the magnetizing voltage. When a transformer is first energized, a transient current up to 10 to 50 times larger than the rated transformer current can flow for several cycles. This current is called as magnetizing inrush current. Magnitude of this current is dependent on number of parameters like switching instant of supply voltage, residual flux, the hysteresis characteristics of the transformer core, impedance of the primary circuit, etc. This magnetizing inrush current causes system disturbances and damages the transformer windings. To improve this situation inrush current is required to be reduced. This paper discusses inrush current limiters that reduce inrush current at the time of switching of the transformer. Here, inrush current limiters using power electronic converters like diode bridge with DC reactor and voltage source PWM converter are used. Simulations using MATLAB are carried out and results are tabulated. Experiment using DC reactor type inrush current limiter are carried out and results are given in the paper.

Manisha Wani

Papers

Use of power electronic converters to suppress transformer inrush current